1. Field of the Invention
The present invention relates generally to laser soldering. More particularly, the present invention relates to mounting pad configurations for attaching electronic surface mount components to substrates using laser soldering.
2. Disclosure Information
Surface mount electronic components are typically attached to a printed circuit board (PCB) substrate by reflow soldering. In this process, (1) the mounting pads on the PCB are solder pasted, followed by (2) placement of the component atop the PCB with the component's terminations registered atop their respective solder-pasted mounting pads, followed by (3) sending the assembly through a reflow oven in which the solder paste is melted and allowed to cool so as to form solid solder joints connecting each component termination with its respective mounting pad.
An alternative to reflow processing is laser soldering, particularly diode laser soldering. Diode laser soldering is particularly useful when the substrate is molded from a low-melting-point polymer, such as polyethylene or polypropylene, because the diode laser frequency (typically 900-950 nm) is such that the laser energy is easily absorbed by metals, such as solder, but only negligibly absorbed by plastics, such as the substrate. Like reflow soldering, laser soldering includes the steps of solder pasting the PCB mounting pads and placing the component with its terminations resting atop their respective pasted pads. However, instead of sending the PCB through a reflow oven, laser soldering involves directing one or more laser beams at the pads and/or terminations in order to heat up and melt the solder to form the solder joints.
FIGS. 1 and 2 show two typical applications of laser soldering according to the prior art (and according to U.S. Pat. No. 4,926,022 to Freedman, in particular), involving a surface mount component 50 having terminations 54 thereon, and a PCB substrate 10 with mounting pads 12 on the top surface 11 thereof. The conventional practice for laser soldering is to either (1) configure the mounting pads 12 to be approximately the same size and shape as would be the case for reflow soldering and to direct the laser beam onto the termination 54 as it rests atop its mounting pad 12, as illustrated in FIG. 1, or (2) size the mounting pad 12 so as to include a portion 13 extended outward beyond the normal pad size used in either reflow processing or (1) above, so that the laser beam may be directed onto the extended portion 13 rather than directly onto the component terminations 54, as shown in FIG. 2. The advantage of utilizing the latter process is that the laser only indirectly heats the component termination (via conduction through the pad 12 and solder), thereby reducing the likelihood of damaging the component.
If this process were applied to components 50 which have a heat spreader 52 on the bottom surface thereof, such as a DPAK, D2PAK, or D3PAK, a configuration such as that illustrated in FIG. 3 would result. As shown, each termination mounting pad 12 and heat spreader mounting pad 20 would have a respective extended portion 13/25 onto which the laser beam B may be directed. Although the prior art approach of lasing the extension 13 may be effective for the relatively small termination pad 12, the heat spreader mounting pad 20 and extension 25 are so large and thermally massive that a considerable amount of laser energy is required to get enough heat to the solder paste underneath the heat spreader 52 in order to melt it and form a solder joint bonding the spreader 52 to the pad 20. The large amount of laser energy needed to form the heat spreader solder joint in the configuration shown may be so large as to deform, distort, stress, melt, degrade, or otherwise deleteriously affect the underlying substrate 10, especially if low-melting-point polymers are used to construct the substrate.
It would be desirable, therefore, to provide a way of laser soldering components having heat spreaders thereunder in such a way that the underlying substrate is not exposed to high levels of potentially damaging heat.